1. Field of the Invention
The present invention generally relates to a color image forming apparatus, a method, and a computer program product for a driver controller of the color image forming apparatus, in which a plurality of electrophotographic image forming units are arranged along a conveying direction of an endless belt.
2. Description of the Background Art
There are generally two kinds of image transfer systems. One of them is a direct transfer system that transfers different color toner images formed on plural photosensitive drums directly onto a transfer paper, as a representative method, for color image formation. The other is an intermediary transfer system that transfers different color toner images formed on plural photosensitive drums onto an intermediary transfer device, and after that transfers all the toner images onto a transfer paper at the same time.
Image forming devices of the direct and intermediary transfer systems are referred to as Tandem type because plural photosensitive drums are arranged at an opposite side in a transfer paper or an intermediary transfer device. The Tandem type devices execute plural process such as formation of an electrophotographic image for each color of magenta (M), cyan (C), yellow (Y), and black (K) on each of respective photosensitive drums, and execute an electrophotographic process of developing. The Tandem type devices transfer images directly onto a transfer paper in the direct transfer system, and transfer images to an intermediary transfer device in the intermediary transfer system.
In a color image formation device of such a Tandem type system, a direct transfer system uses an endless belt as a paper conveyance belt running for supporting a transfer paper, and an intermediary transfer system uses an endless belt as an intermediate transfer belt for forming images thereon from photosensitive drums. In general, an image process unit including four photosensitive drums is set along the conveying direction of the endless belt. In such a structure, it is very important to ensure that each individual color image is overlaid preciously at the same position on either the transfer paper or the intermediate transfer belt, and thus it is important to control a position alignment technique between each color image in a color image forming apparatus of the Tandem system.
Various systems have conventionally been implemented for maintaining such a positional alignment for each color image.
For example, there is known a controlling method that forms toner patterns for adjusting each color on the endless belt and detects with a sensor a color misalignment and adjusts a writing timing of the optical writing unit based on any detected color misalignment.
In such a background art, for example Japanese Laid-Open patent Publication No. 2001-215857 (the entire contents of which are hereby incorporated herein by reference) describes that a color misregistration is prevented by changing a conveyance speed of a transfer device conveyance depending on a lap length of a transfer conveyance device resulting from an environmental change and a temperature rise in a device (endless belt).
In a color image formation device of a so-called Tandem system, color misregistration can easily result between each of the colors to be formed. The main causes of the color misregistrations are Skew difference, a registration difference in a sub-scanning direction, a magnification error in a main-scanning direction, a registration difference in a main-scanning direction, etc.
The above-mentioned toner pattern system is described as follows referring to FIG. 11. FIG. 11 shows toner pattern lines 201 for a color adjustment formed on an endless belt 200, which e.g. is a sheet conveyance belt. In the toner pattern lines 201, horizontal lines and diagonal lines of each of colors K, Y, C, M are formed and these pattern lines are detected by sensors 202-204 set in a main-scanning direction. Using outputs of the sensors 202-204, an amount of a skew difference, a registration difference in a sub-scanning direction, a registration difference in a main-scanning direction, and a magnification difference in a main-scanning direction relative to a main color (in this case, Black (K)), are all calculated. Based on the calculation results a main CPU makes a correction of each color misalignment.
A correction of a skew difference is realized by modifying a slope of a mirror that reflects the laser light of each color in an optical writing unit. A stepping motor is used with a driver to change the slope of the mirror. The corrections of registrations in a main-scanning and in a sub-scanning directions are realized by adjusting a start-writing timing. Also, based on the result of a mark detection and calculation, when there is a misalignment of the main color about a magnification in a main-scanning direction, the device can change the frequency with a very small step, for example a clock generator can change the magnification.
However, when such a toner pattern system forms toner patterns of each color and makes an automatic color detection by detecting these patterns, down time of the machine occurs and copy productivity will be lost. Especially considering the color difference in a sub-scanning direction, the speed change of an endless belt should be considered, and actually toner pattern lines 201 shown in FIG. 11 are formed plural times (for example, eight sets are formed) in a sub-scanning direction. For calculating an average of the results of the detection, a toner pattern is formed by merely a timing of paper intervals, and thereby a long down time of a machine in such conditions cannot be avoided. The term “down time” in this context indicates a time that the device in FIG. 11 needs to form the toner patterns 201 and to calculate any color differences based on detecting those toner patterns 201 with the sensors 202-204. That is, forming the toner patterns 201 and processing data from detecting the toner patterns 201 requires time during which image formation cannot be effectuated, resulting in a down time in image formation.
The above patent document JP 2001-215857 discloses changing a lap length of an endless belt that occurs by a temperature rise, and the conveyance speed of an endless belt is controlled according to the change of the lap length. However, it is not enough to correct a color misalignment only paying attention to the change of a lap length of an endless belt because a color misalignment in a sub-scanning direction results not only from a change of a lap length of an endless belt, but also by results compounded from an influence of an optics device in an optical writing unit, a speed change of the endless belt, a stretching of the device housing itself, etc.